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One of the major problems in the liquid insulation is the particle contamination, which can seriously degrade the withstand voltage of the insulation. The movement of conducting particles under operation voltage can generate incipient discharges that can lead to the complete failure of the insulation system during operation. The partial-discharge characteristics of the free spherical conducting particle in the quasiuniform electric field were studied in this paper in order to better explain the partial-discharge (PD) measurement signals for particle movement and implications for insulation diagnostics in power transformers. The phase-resolved partial-discharge (PRPD) pattern was measured and the PD inception voltage, PD charges, and PD frequency were studied under different applied voltages. The influence of oil aging degree to particle PD characteristics was also studied in this paper. It is observed that the PD pattern is different in a different particle motion stage and the number of particles has a significant influence on PD frequency and a slight influence on PD pattern. The particle is inactive in severely aged transformer oil. The PD charges and PD frequency (pulses number per half ac cycle) reduce with the aging degree of the oil sample.